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Bedrock Uplift from Greenland’s Peripheral Glaciers

Posted by William Colgan on January 16, 2024
Climate Change, Communicating Science, New Research / No Comments

We have a new article in the current issue of Geophysical Research Letters that looks at the influence of Greenland’s peripheral glaciers on vertical bedrock motion. Greenland’s bedrock is currently uplifting, due to both slow mantle-deformation processes associated with ice loss at the end of the Last Glacial Period, and fast elastic processes associated with ice loss today. The vertical bedrock uplift being measured in Greenland today ranges from a couple millimeters to a couple centimeters across the country. Understanding the magnitude and spatial distribution of this uplift helps us understand not only recent ice loss, but also properties of the Earth’s mantle beneath Greenland.

Figure 1 – Time series of the observed vertical land motion (VLM) at Mestersvig (MSVG) station in East Greenland. The elastic rebound associated with the Greenland Ice Sheet (GrIS), Greenland Peripheral Glaciers (GrPG) and Canadian Peripheral Glaciers (CanPG) are calculated. The post-Last Glacial Period glacioisostatic adjustment (U_GIA) is then calculated as a residual.

When folks create maps of Greenland’s present-day uplift rate, they typically use a model of changing ice-sheet geometry through time, to incorporate the effect of changing ice load on the Earth’s crust. This captures the main signal, but it ignores the cumulative effect of Greenland’s thousands of peripheral glaciers. These glaciers, which surround the ice sheet, also effect vertical bedrock motion. In this study, we also incorporate the effect of changing peripheral glacier geometry through time into uplift rates calculated at all the GNET bedrock motion sites around Greenland. In the figure above, you can see the vertical land motion budget of MSVG (Mestersvig) GNET station, which calculates post-Last Glacial Period glacioisostatic adjustment (GIA) as the residual of present-day elastic rebound.

Figure 2 – Comparison between post-Last Glacial Period glacioisostatic adjustment (GIA) that we calculate across the 58 GNET stations, compared to four widely used maps of Greenland GIA. A mismatch between the station color and the map color highlights a discrepancy between the previously calculated GIA and the GIA calculated in this study. These four previous studies used different methods, but all ignored the elastic rebound associated with peripheral glaciers.

We find that peripheral glaciers can have a disproportionately large impact on the elastic rebound of GNET sites, especially when they are located relatively far from the ice sheet. In some regions, especially in Greenland’s north and northeast, peripheral glaciers can contribute to over 20% of the total elastic response of regional GNET sites. Simply put, mapping Greenland’s present-day uplift rate with models that only incorporate the ice sheet, and not peripheral glaciers, can really underestimate the elastic rebound associated with present-day ice loss. Under estimating present-day elastic rebound can result in subsequently over estimating the post-Last Glacial Period glacioisostatic adjustment that is used to infer mantle properties.

So, perhaps the main message of this study is that although Greenland’s peripheral glaciers are quite small in comparison to the ice sheet, their recent collective ice loss can influence our understanding of Greenland’s vertical land motion in a disproportionately large way!

Open-Access Study: Berg, D., Barletta, V. R., Hassan, J., Lippert, E. Y. H., Colgan, W., Bevis, M., et al. (2024). Vertical land motion due to present-day ice loss from Greenland’s and Canada’s peripheral glaciers. Geophysical Research Letters, 51, e2023GL104851. https://doi.org/10.1029/2023GL104851

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Greenland Ice Sheet mass loss from combined CryoSat-2 and ICESat-2 altimetry

Posted by William Colgan on March 31, 2022
Climate Change, New Research, Sea Level Rise / No Comments

We have a new open-access study out in the current volume of Journal of Geophysical Research that brings together both radar and laser altimetry measurements to assess the mass balance of the Greenland Ice Sheet between 2011 and 2020. Our assessment shows that the ice sheet lost approximately 498 Gt of ice volume, corresponding to approximately 7 mm of global sea-level equivalent during this time. The peak loss year was from April 2019 to April 2020, when the ice sheet lost 1.4 mm of global sea-level equivalent, which is equivalent to losing 15,850 tonnes of ice per second for an entire year. These values reflect only the ice sheet proper, and ignore Greenland’s peripheral glaciers. 

Figure 1 – Annual mass loss, partitioned into meltwater runoff (SMB) and iceberg calving (Dynamic) components, across the eight major ice-sheet sectors during 2011-2020. Central map shows the average ice volume change over 2011-2020 resolved from both CryoSat-2 and ICESat-2 altimetry measurements. Individual glaciers indicated: Jakobshavn Isbræ (JI), Helheim Glacier, Kangerlussuaq Glacier, Nioghalvfjerdsfjorden Glacier, the Zachariae Isstrøm, Storstrømmen Glacier, Petermann Glacier, Humboldt Glacier, and Northeast Greenland Ice Stream.

Over the study period, we estimate that approximately 43% of the ice loss was due to ice dynamics (i.e. more iceberg calving). Surface mass balance, or meltwater runoff, was responsible for the remaining approximately 57% of mass loss. This partition of mass loss varies tremendously between ice-sheet catchments and through time. This allows us to see the ice-sheet responding to recent climate forcing at the scale of individual seasons and catchments. Our assessment even resolves mass loss changes at the level of individual glaciers. For example, we can see that after a brief period of ice thickening during 2018 and 2019, Jakobhavn Isbræ has now returned to substantial ice thinning. 

If you’re really into satellite altimetry, we also make a rather unique cross-comparison between ICESat-2 laser measurements and CryoSat-2 radar measurements. While laser altimetry enjoys a near-complete surface scattering of the incoming laser pulse, radar altimetry has substantial volume scattering, meaning that the incoming radar pulse penetrates some depth into the ice sheet. This makes it difficult to assimilate both laser and radar altimetry measurements into a common processing pipeline. But, after applying the necessary volume-scattering correction to the radar measurements, we can assimilate both the ICESat-2 and CryoSat-2 observations into a common framework that shows good agreement (±8 cm/yr) during the common 2019/20 year.

Figure 2 – Ice-sheet volume change over the April 2019 to April 2020 period from ICESat-2 laser altimetry (A), CryoSat-2 radar altimery (B) and their difference (C). This year of peak mass loss, during which time the ice sheet was well-sampled by both altimeters, saw a record ~498 Gt of ice loss from the ice sheet.

Finally, to be consistent with an open science mandate and help the community move forward as fast as possible, we make the annual (April to April) ice-sheet maps of volume change that we assess at 1 km spatial resolution available for download at: https://datadryad.org/stash/share/gRoJh1JfpF4EA1d_Prsa_KIju9z2hJXWvXE5J1X2d8I. We hope these data will be useful for not only inter-study altimetry comparisons, but also for initializing models that calculate the elastic uplift of Greenland’s bedrock and evaluating ice flow models that simulate recent ice-sheet mass loss. 

Khan, S. A., Bamber, J. L., Rignot, E., Helm, V., Aschwanden, A., Holland, D. M., van den Broeke, M., King, M., Noël, B., Truffer, M., Humbert, A., Colgan, W., Vijay, S., and Kuipers Munneke, P. (2022). Greenland mass trends from airborne and satellite altimetry during 2011–2020. Journal of Geophysical Research: Earth Surface. 127. e2021JF006505. https://doi.org/10.1029/2021JF00650

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